从DATETIME中选择每年的每月最后插入值

Question

I got a DATETIME to store when the values where introduced, like this example shows:

CREATE TABLE IF NOT EXISTS salary (
    change_id INT(11) NOT NULL AUTO_INCREMENT,
    emp_salary FLOAT(8,2),
    change_date DATETIME,
    PRIMARY KEY (change_id)
);

I gonna fill the example like this:

+-----------+------------+---------------------+
| change_id | emp_salary |     change_date     |
+-----------+------------+---------------------+
|         1 |     200.00 | 2018-06-18 13:17:17 |
|         2 |     700.00 | 2018-06-25 15:20:30 |
|         3 |     300.00 | 2018-07-02 12:17:17 |
+-----------+------------+---------------------+

I want to get the last inserted value of each month for every year.

So for the example I made, this should be the output of the Select:

+-----------+------------+---------------------+
| change_id | emp_salary |     change_date     |
+-----------+------------+---------------------+
|         2 |     700.00 | 2018-06-25 15:20:30 |
|         3 |     300.00 | 2018-07-02 12:17:17 |
+-----------+------------+---------------------+

1 won't appear because is an outdated version of 2

Answer 1

You could use a self join to pick group wise maximum row, In inner query select max of change_date by grouping your data month and year wise

select t.*
from your_table t
join (
  select max(change_date) max_change_date
  from your_table
  group by date_format(change_date, '%Y-%m')
) t1
on t.change_date = t1.max_change_date

Demo

If you could use Mysql 8 which has support for window functions you could use common table expression and rank() function to pick row with highest change_date for each year and month

with cte as(
    select *, 
    rank() over (partition by date_format(change_date, '%Y-%m') order by change_date desc ) rnk
    from your_table

)

select * from cte where rnk = 1;

Demo

Answer 2

The below query should work for you. It uses group by on month and year to find max record for each month and year.

SELECT s1.*
FROM salary s1
INNER JOIN (
    SELECT MAX(change_date) maxDate
    FROM salary
    GROUP BY MONTH(change_date), YEAR(change_date)
) s2 ON s2.maxDate = s1.change_date;

Fiddle link : http://sqlfiddle.com/#!9/1bc20b/15